This invention concerns a method to measure the variations in section of a rolled bar.
The invention can be applied substantially in every type of rolling and/or finishing process, on hot bars as they are being fed at high and very high speed, both in the inter-stand segments and also at the inlet or outlet of a rolling train.
The invention makes it possible to monitor the variations in the area of the normal section at the rolling axis of the bar as it is fed, and to measure the entity thereof, in order to determine, in an extremely precise and substantially instantaneous manner, whether there is any difference compared to the programmed nominal area.
Moreover, the invention makes it possible to take speedy correction measures, for example adjustments and modifications to the clearances of the stands upstream or downstream or of the inter-stand drawing force if the variations monitored exceed allowed and pre-defined values of tolerance.
One of the most common problems in hot rolling processes, particularly in the production of long products and/or in plants where rolling takes place directly in line with the casting, is that it is necessary to ensure that the area of section of the bar being rolled is maintained at the programmed nominal values, within a tolerance value which is allowed and can be pre-set in advance, for the whole of its lengthwise extension.
For it is known that deformations and reductions of the bar can cause variations in section of the bar which make the bar, or sections thereof, exceed an allowed tolerance of size; these deformations and reductions can be caused by such factors as: an inconstant and disuniform drawing force, progressive wear of the rolls, an incorrect action of the drawing assemblies, lengthwise deformities in the composition of the material, variations in temperature or other factors.
This can cause an unacceptable deterioration in the quality of the material and it may be necessary to resort to auxiliary finishing processes and even to discard a large quantity of the product.
The state of the art covers U.S. Pat. No. 4,607,511 which includes systems to measure the size of the rolled product in transit so as to identify in an indirect manner the value of the inter-stand drawing force to which the rolled product is subjected.
U.S. Pat. No. '511 includes an optical device to measure the diameter located at the outlet of the rolling stand downstream and a similar optical device located at the inlet of the rolling stand upstream.
These devices are very delicate and must be protected, particularly from water and steam, so that their structure and functioning are not compromised.
Moreover, using the measurement of the diameter to obtain the area of section of the bar in transit involves extremely complex calculations, which require a long time before the result is obtained.
JP-A-089124 includes a device to measure the diameter which measures the dimensional pulses of the rolled product in transit; however, if dimensional pulses are perceived, this means that the product is already beyond the tolerance of size and therefore no longer acceptable.
Devices to measure diameter which are known to the state of the art are generally delicate in functioning and therefore not very suitable to working environments such as those of a rolling mill, they do not give a direct and therefore instantaneous measurement, on the contrary they require a long time and complex calculations to obtain the result, and moreover they are very often easily influenced by external disturbances.
U.S. Pat. No. 4,083,002 and DE-A-24 35 338 describe measuring devices comprising substantially a first solenoid which coaxially surrounds a metallic bar as it is fed, and generates a magnetic field which uniformly fills its free inner space, and at least a second coil inside the first solenoid but outside and coaxial to the bar.
The passage of the bar causes a displacement of the lines of force of the magnetic field produced by the first solenoid, which displacement is monitored by the second coil.
By measuring the signals induced on the second coil, it is possible to obtain a measurement of the transverse section of the advancing bar.
Although these devices known to the art are appreciated in themselves because they perform sensitive measurements, they have shown that they lack accuracy inasmuch as they are greatly influenced by the variations in the process parameters, and particularly by the temperature of the bar which is to be measured
The present applicant has found that when measuring the sections of metallic bars at different temperatures there are great differences in the values monitored, and therefore these measurements cannot be considered completely reliable in a wide range of situations and particularly on bars leaving a hot rolling mill, which can have quite different temperatures.
The present applicant has designed, tested and embodied this invention to overcome the shortcomings of the state of the art and to achieve further advantages.